Pentax 50-135mm f2.8 or Sigma 70-200mm f2.8???

I just recently purchased a K5 body with a 18-135mm f3.5-5.6 lens knowing I would be looking to upgrade the glass. I'm trying to decide between the pentax 50-135 mm f2.8 which would provide the full frame equivalent of 70-200mm or the Sigma 70-200mm which would provide a slightly longer telephoto ability.

Thoughts on either lens? Looking for something good to capture the kids sports!